Light-harvesting pigment molecules chlorophylls

The photosynthetic apparatus is found in and on membrane structures, which, in plant cells and algae, are located in chloroplasts and are called thylakoids. In bacteria the photosynthetic membrane is derived by complex invagination of the cytoplasmic membrane. The photosynthetic apparatus is made up of antennae, which contain light-harvesting pigment molecules (usually chlorophylls or bacteriochlorophylls) and photochemical reaction centres, which also contain pigments, together with the necessary enzymes and coenzymes. 

The unique water-soluble peridinin- Chi a-protein (PCP) complexes are found in many dynoflagellates in addition to intrinsic membrane complexes. [64] It contains Chi a and the unusual carotenoid peridinin in stoichiometric ratio of 1 4. Unlike other families of antennas, the main light-harvesting pigments are carotenoids, not chlorophylls. The structure of the PCP consists of a protein that folds into four domains, each of which embeds four peridinin molecules and a single Chi a. The protein then forms trimers, suggested to be located in the lumen [64] in contact with both LHCI and LHCII [66], allowing efficient EET to occur. 

The two photochemical reactions are performed by two photosystems. Each photosystem consists of a so-called reaction centre, where the primary energy conversion takes place, associated with a few hundred pigment molecules (chlorophylls and carotenoids see Fig. 2) serving as light-harvesting antennas, which transfer the absorbed energy as electronic excitation energy to the reaction centres. 

The first part of the process occurs in light-harvesting complexes. Each multisubunit protein complex contains multiple antenna pigment molecules, chlorophylls and some accessory pigments, and two chlorophyll molecules that act as the reaction center. The reaction center traps energy quanta excited by the absorption of light. 

FIGURE 19-42 A light-harvesting complex, LHCII. The functional unit is an LHC trimer, with 36 chlorophyll and 6 lutein molecules. Shown here is a monomer, viewed in the plane of the membrane, with its three transmembrane a-helical segments, seven chlorophyll a molecules (green), five chlorophyll b molecules (red), and two molecules of the accessory pigment lutein (yellow), which form an internal cross-brace. 

---